There might be two "large size" sockets empty on the "innermost" sides of each light fixture. In fact, those sockets are so empty they do not have electrical connectors in them. These sockets are for fairly large rear fog light bulbs. In reading here it seems that in Europe one rear side will have a fog light bulb, depending on which side of the road they drive on.

If there are two "small size" sockets that are empty on the "outermost" sides of each light fixture, those are probably for the 10-watt marker lights. They should be there - check for electrical connectors inside these sockets.

Previous owner probably too lazy to replace one blown so they took them both out.

They make the car a little safer because these marker lights can be seen from the side.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles